The present invention is related to a loudspeaker system and a sound producing apparatus, specifically to a loudspeaker system and a sound producing apparatus advantageous for use in a car.
In the era of popular use of Compact Disk(CD), Mini Disk(MD), Digital Audio Tape(DAT) and other digital sound sources, loudspeaker systems are requested to be able to reproduce the recorded sound for a wide range. In order to improve the low range sound reproduction, Kelton type speakers have been known, in which a speaker unit and a passive radiator are combined.
In the following, a conventional Kelton type speaker system having passive radiator is described referring to Fig.14. The conventional speaker system comprises a passive radiator 101 which actually produces a sound, a speaker unit 102 for driving the passive radiator 101, a baffle 103, being a constituent of speaker box, for mounting the passive radiator 101 on, a front closed cavity 104 for coupling the sound output of speaker unit 102 with the passive radiator 101, a back closed cavity 105 for sealing the backward sound output of speaker unit 102 in, a sub-baffle 106, which splits a space into the front closed cavity 104 and the back closed cavity 105, for mounting the speaker unit 102 on, and a cabinet 107 which constitutes a speaker box in combination with the baffle 103. The sub-buffle 106 is fixed to the side wall of speaker box.
FIG. 15 is an equivalent circuit diagram of a conventional Kelton type speaker system having passive radiator. Represented in FIG. 15 are: an electromagnetic resistance 201 due to reverse electromotive force of speaker unit etc., a mechanical resistance 202 of speaker unit, an equivalent mass 203 due to the mass of speaker diaphragm etc., a compliance 204 due to damper and edge etc. of speaker unit, a transformer 205 conducting the sound output of speaker unit to a passive radiator in proportion to the area ratio, a mechanical resistance 206 of passive radiator, a compliance 207 due to damper and edge etc. of passive radiator, an equivalent mass 208 due to the mass of passive radiator diaphragm etc., a compliance 209 due to front closed cavity, and a compliance 210 due to back closed cavity.
Now in the following, description is made on the operation of a conventional speaker system having the above described constitution. A sound is produced, as illustrated in FIG. 14, by a sound producing output discharged from the front of speaker unit 102, which drives the passive radiator 101 mounted on baffle 103 by means of the air existing within front closed cavity 104 formed by baffle 103 and sub-baffle 106; the passive radiator 101 actually produces a sound. A sound producing output discharged from the back of speaker unit 102 is sealed within the back closed cavity 105 formed by sub-baffle 106 and cabinet 107 so as it does not interfere with the sound producing output of passive radiator 101.
FIG. 16 is a comparison of low range sound producing characteristics, showing the advantages of a conventional Kelton system having passive radiator. In FIG. 16, a line 301 represents the frequency characteristics of output sound pressure level in a sealed cabinet system. In the sealed cabinet system, sound producing output generated from the back of a speaker unit is sealed within a speaker box in order to avoid the interference with the sound producing output from the front of speaker unit. If a speaker box is not sufficiently large, the compliance with respect to the speaker unit deteriorates, and the low range producing capability is limited, as indicated by the shape of line 301.
A line 302 represents the frequency characteristics of output sound pressure in a reversed phase system using a same type speaker unit and speaker box. In the reversed phase system, a sound producing output generated from the back of a speaker unit is made to resonate through a duct of the speaker box at a certain frequency(hereinafter referred to as anti-resonance frequency), which is mixed with a sound producing output from the front of speaker unit. The sound producing output through the duct has a same phase as that from the front of speaker unit at the vicinity of the anti-resonance frequency, which improves the radiation efficiency by mutual effects and the limit of low range sound is extended as compared with the sealed cabinet system. In a very low frequency range, however, the phase of sound producing output through the duct is reversed against the sound producing output from the front of speaker unit, which invites the mutual offsetting. As a result, the characteristics curve shows a steep decrement of approximately -20 dB/oct. in the very low frequency range. Therefore, heavy bass production remains unsatisfactory.
A line 303 represents the frequency characteristics of output sound pressure in a conventional Kelton system having passive radiator using a same type speaker unit and speaker box. In the same manner as in the reversed phase system, the Kelton system makes passive radiator, speaker unit and speaker box resonate at a certain frequency(hereinafter referred to as lowest resonance frequency), extending the limit of low range sound production. As the passive radiator and the speaker unit resonate in a same phase, the decrement at very low frequency range follows a same moderate curve approximately -12 dB/oct. as that of the sealed cabinet system, producing a sufficient heavy bass. Furthermore, the passive radiator does not vibrate in a frequency range higher than a certain frequency despite a vibrating speaker unit, which means that the system is provided also with an excellent band pass characteristic as a low range speaker.
As described above, a conventional Kelton system having passive radiator improves the low range sound production providing the advantages of both the sealed cabinet system, good production of heavy bass which is due to moderate decrement characteristic in very low frequency range, and the reversed phase system, an extended limit of low range production.
The conventional Kelton system having passive radiator is a speaker system quite effective in improving the low range production, as described above. In the system, however, the sub-baffle for splitting a space inside the speaker box into a front closed cavity and a back closed cavity is mounted on it with a speaker unit, being a vibration source, and is fixed to the side wall of speaker box, which makes the sub-baffle itself a source of unwanted vibration due to the counteraction of speaker unit. This accompanies a problem, that is a larger unwanted vibration of a speaker system itself.
Another problem with the system is that in a compact or flat configuration it is difficult to obtain an ideal sound characteristic because of the structural limitation in reducing the volume of front closed cavity; the reason is, one of the parameters which give a substantial influence on the sound characteristics of this type of speaker system is the ratio in volume between front closed cavity and back closed cavity, and there is a limitation in making the volume of front closed cavity small in a structure where a space is splitted into front closed cavity and back closed cavity by a sub-baffle.